Technical Field
The present invention relates generally to audio amplification. More particularly, the invention relates to devices, systems and methods for multi-channel amplification in class D amplifiers.
Background Art
Typically one of the last components in an audio distribution chain, audio amplifiers amplify a low power audio signal to a level suitable for driving one or more loudspeakers. Multi-channel audio amplifiers are employed throughout structures to amplify more than one channel of audio.
As known to those of skill in the art, typical Class D amplifiers are a class of amplifier in which the audio signal modulates a pulse width modulated carrier signal to drive the output. Referring now to FIG. 1, a block diagram of class-D amplifier 100 is illustrated showing the different stages of operation of the amplifier. Input audio signal (input signal) 102 is input to pulse width modulator (PWM) comparator 104, which is part of modulator block 109, along with triangle wave 105 that has been generated by triangle wave generator 103. In PWM comparator 104, input signal 102 is compared to triangle wave 105. Triangle wave (or sawtooth wave) 105 is typically a much higher frequency signal than input signal 102—usually ten or more times the highest expected frequency of input signal 102. Thus, if an audio signal of about 20 Hz to about 20 kHz is expected as input signal 102, then triangle wave 102 should be at least about 200 kHz. As its name implies, PWM comparator 104 typically includes a comparator, and compares the amplitudes of input signal 102 to that of triangle wave 105 to form a pulse width modulated output. That is, the width of each pulse will be dependent upon the amplitude of both input signal 102 and the amplitude of triangle wave 105, but at a frequency equal to that of triangle wave 105. The duty cycle is proportional to the amplitude of input signal 102. If both input signal 102 and triangle wave 105 are centered about 0 volts, then for a 0 volt input signal 102, the output duty cycle is about 50%, and if input signal 102 is about Vtmax, the maximum of triangle waveform 105, the duty cycle is about 100%. If input signal 102 is about Vtmin, the lowest voltage value of triangle waveform 105, then the duty cycle is about 0%. FIG. 2 illustrates an example of a typical pulse width waveform generated in the above-described manner.
Referring now to FIG. 2, input signal 102 is shown as a roughly sinusoidal shaped signal with a frequency of about 1/20th of that of triangle waveform 105. Comparator stage output 106 is shown below input signal 102 that is super-imposed over triangle waveform 105; the logic level “high” represents the times in which input signal 102 exceeds that of triangle waveform 105. It can be seen that at points A and B, the duty cycle of comparator stage output 106 (the pulse width modulated signal) about 50%, and as input signal 102 swings closer to Vtmax (at point C) and Vtmin (at point D), the duty cycle of comparator stage output 106 changes from about 100% to about 0%, respectively.
As known by those of skill in the art, typical class D amplifiers with multiple channels typically comprise a global triangle ramp generator for use in all of the channels. A global analog buffer and a local analog buffer can be inserted between each channel and the triangle ramp generator.
Following PWM comparator 104, comparator stage output 106 is input to switching output stage (or power stage) 108, which is also part of modulation block 109. This device is typically comprised of an arrangement of switching transistors configured as a “half-bridge” or “full-bridge” and it amplifies the signal input to it, to create switching output signal 110. As can be seen in FIG. 1, switching output signal 110 is an amplified version of comparator stage output 106, and switches between only two states, the positive and negative power supply rail voltages (in the case of the “half-bridge” implementation), and the positive power supply rail voltage and ground (in the case of the “full-bridge” implementation). As known to those of skill in the art, the gain of the typical Class D amplifier modulator stage is set by the ratio of the power supply voltage, (or at least the power supply voltage that is available at switching output stage 108) and the peak-to-peak triangular wave voltage (triangle waveform 105). For example, the gain of the modulator stage, of a half-bridge implementation with +/−50VDC power supply rails, and a triangular wave voltage of 10Vpp is 100V/10V=10.
Following switching output stage 108 is filter stage 112. In filtering stage 112, the amplified PWM signal is passed through an (ideally) lossless low pass filter prior to the output device, speaker 116. The low pass filter removes the high frequency components of the PWM signal (switching output signal 110) and recovers the original audio signal, but in an amplified form, now referred to as amplified output signal 114.
Having briefly reviewed operation of a Class D amplifier in a fairly general sense, attention can now be directed to specific design issues with regard to Class D amplifiers. As those of skill in the art can appreciate, there are a multitude of design issues that need to be carefully considered with each new design of a Class D amplifier. Two such design considerations shall be considered herein. The first is isolation between channels, and the second is the safe operation over different load impedances.
Isolation between channels is a critical design consideration for multi-channel audio amplifiers. This isolation is typically expressed in decibels (dB) at a specific frequency, and further is typically a fairly small signal, thus a negative dB rating is typical, as the crosstalk signal is almost always much less than the original signal. Poor channel-to-channel isolation results in the audio signals from one channel being heard in another channel, which can result in poor channel separation. In a typical audio system, crosstalk can be audibly heard when volume levels are low (if there is a cross talk problem, which is not always the case). Notwithstanding its noticeability only when at low audio volumes, crosstalk, as mentioned above, can negatively affect channel separation, which could become more noticeable even at normal volume levels. Accordingly, high isolation (i.e. higher-dB crosstalk ratings) is desirable in multi-channel amplifiers.
One conventional solution to reduce channel-to-channel crosstalk includes the careful design of the printed circuit board (PCB) layout. Grounding may be used to eliminate common-impedance traces and mixing of signal and/or ground currents from more than one channel. Another scheme employed in conventional Class D amplifiers is differential signal routing instead of routing single-ended signals with a ground potential that is common to all channels. However, whenever a power, ground, or signal is common to more than one channel, it becomes a potential conveyer of crosstalk.
There are certain problems, however, with many of the conventional solutions to reduce crosstalk described above. Accordingly, it would be desirable to provide methods, modes and systems for reducing crosstalk and its effects in multi-channel Class D audio amplifiers.
The second design consideration to be considered herein is safe operation of a Class D amplifier over different load impedances. In Class D amplifiers, as known to those of skill in the art, the power conversion efficiency of the amplifier is only slightly degraded as DC rail voltages are increased. This decrease in efficiency is attributable to increased switching losses. In contrast, however, on-state losses actually decrease as the DC rail voltages rise (due to reduction in MOSFET ON-TIME (duty-cycle). Thus, the Class-D topology already provides more freedom in the selection of DC rail voltages to power the half-bridge or full-bridge stage (i.e., switching output stage 108). An additional important effect in selecting DC rail voltages is that the audio output signal will be able to span the entire peak-to-peak range defined by the DC rail voltage presuming a full duty cycle range from 0% to 100%. Therefore, as those of skill in the art can appreciate, increasing the DC rails will allow greater output voltage, current, and power (for a given load impedance).
There are applications where it would be advantageous to set or adjust the DC rail voltages to different voltages in order to tailor the available output voltage, current, or power, to a particular load impedance. Further, it has been alleged to be advantageous to be able to change these DC rail voltages substantially continuously or instantaneously. The dynamic ability to modulate the DC rail voltages, at either audio frequency rates, or static levels set by manual switches, has been attempted by other, conventional systems. At least one disadvantage of this prior art method is increased complexity in power supply design, and the additional complexity in the means to modulate the power supply output voltage(s). Another disadvantage to this approach, is that the modulator gain of the Class-D amplifier is directly affected by the magnitude of the DC rails, as previously discussed, and therefore the open-loop gain of the Class-D amplifier channel is directly affected by the magnitude of the DC rails. This can cause instability in the negative feedback loop compensation of the amplifier, as will be discussed further below.
Accordingly, it would be desirable to provide methods, modes and systems for enabling a Class D amplifier to be connected to different load impedances without having to change or vary the power supply voltage to provide safer operation.